4ohm vs 8 ohm......why one or the other.......

typically a speaker with a higher impedance is more desired than one with a low impedance, especially if you have a tube amp - which will sound like it has tighter bass if your speaker has a 12 OHM load instead of a 4 OHM load for example.

A tube amp does not change power output when speaker impedance changes like a solid state amp does, when you lower impedance into a solid state amp they put out more power but a tube amp usually puts out about the same amount of power when the impedance changes.

Usually speaker impedance is more important when you have a tube amp.

This is a very rough answer and only a guideline as there are exceptions - and certainly better answers - I am sure.

.....Garfish, some speaker manufacturers (Thiel?) intentionaly correct their crossovers so that the speakers present a fairly consistant load. I believe the intent is to offer amplitude coherence and to help keep the amplifier stable by limiting the electronic gymnastics. Though some claim that these measures suck the life from the over all sound, it seems like a "sound" idea to me.
.....Philojet, high impedance loads can be just as difficult for solid state amps as low impedance loads are to tube amps. In as much as solid state amps tend to put out more power into low impedance loads, the inverse is true as well, solid state amps put out less power into high impedance loads. Most people don't realize that even a mighty Krell with will wince faced with a 32 Ohm load as much as the average (there are always exceptions) tube amp will wince when faced with a 1 Ohm load. Solid state amps don't have as much a problem today, as in yesteryear, when there were many more speakers with loads over 16 Ohms. This is not to say one is better than the other. It's about using the right tool for the job at hand.

There are quite a few well-known combinations of low-impedance speakers that mate very well with tube equipment. Magnepan and MartinLogan speakers both present pretty low impedence loads and both are generally said to sound better with tube equipment. I know my ML's sound even nicer with tube euqipment than with solid state.

The early audio designer had to settle for some figures to work with I guess.
Did it date back to the little note left for the milkman..?
2 pints to a quart, 4 quarts to a gallon...8 pints for an audiophiles' milkshake party?